Which NPC can be pickpocketed with a 100% success rate above level 100?

Solution 1:

You slightly misunderstood the boosting mechanic. You get a permanent +12 boost, but that means other boosts with maximums under +12 don't work. Stews have a max boost of +5 regardless of the skilling prodigy relic. Therefore, the maximum thieving level is 111/99. There is also some reason to suspect that boosts past 99 have no effect on pickpocket success chance. I recall a jmod looking into something similar for mining granite with a dragon pickaxe boost. The jmod decided that boosts past 99 have no effect on mining speed.

The next NPC after Ardougne knights in order of thieving level required are Yanille watchmen at level 65. With 111/99 thieving, the completed Ardougne diary and a thieving cape it is still possible to fail to pickpocket a watchman.